Trona is a naturally occurring sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). The Green River basin in southwestern Wyoming contains the world's largest known deposit of trona. Reserves in Wyoming amount to approximately 140 billion tons. In the Green River Basin there are approximately twenty-five beds of trona more than four feet thick with intervening strata of shale. These beds are encountered at a below surface depth between 500 and 3000 feet.
Trona is the principle source mineral for the United States soda ash industry and is generally produced by conventional underground mining methods. Mined ore is hoisted to the surface and is commonly processed into soda ash either by the ‘sesquicarbonate process’ or the ‘monohydrate process.’ In the sesquicarbonate process, the processing sequence involves underground mining; crushing; dissolving raw ore in mother liquor; clarifying; filtering; recrystallizing sodium sesquicarbonate by evaporative cooling; and converting to a medium density soda ash product by calcining. The monohydrate process involves underground mining, crushing; calcining of raw trona ore to remove carbon dioxide and some organics to yield crude soda ash; dissolving the crude soda ash; clarifying the resultant brine; filtering the hot solution; removing additional organics; evaporating the solution to crystallize sodium carbonate monohydrate; and drying and dehydrating sodium carbonate monohydrate to yield the anhydrous soda ash product.
Solution mining of trona has been proposed to minimize environmental impacts and reduce or eliminate the costs of underground mining, hoisting, crushing, calcining, dissolving, clarification, solid/liquid/vapor waste handling and environmental compliance. The numerous salt (NaCl) solution mines operating throughout the world exemplify solution mining's potential low cost and environmental impact. Attempts to solution mine trona using vertical boreholes began soon after the 1940's discovery of trona in the Green River Basin in Wyoming. U.S. Pat. No. 3,050,290 discloses a process for solution mining of trona that suggests using a mining solution at a temperature of the order of 100° C.-200° C. This process requires the use of recirculating a substantial portion of the mining solution removed from the formation back through the formation to maintain high temperatures of the solution. A bleed stream from the recirculated mining solution is conducted to a recovery process during each cycle and replaced by water or dilute mother liquor. U.S. Pat. No. 3,119,655 discloses a process for the recovery of soda ash from trona and recognizes that trona can be recovered by solution mining. This process includes introduction of water heated to about 130° C. and recovery of a solution from the underground formation at 90° C.
These solution-mining attempts failed to mature. Instead, large-scale traditional underground mines were developed with combined capacity in excess of 17,000,000 tons per year. The most recent borehole trona solution mine attempt involved connecting multiple conventionally drilled vertical wells along the base of a preferred trona bed. Hydraulic fracturing was used to connect the wells. FMC Corp. was the primary company attempting to develop such trona solution mining. FMC staff published a report (E&MJ September 1985 “FMC's Newest Goal: Commercial Solution Mining Of Trona” including “Past attempts and failures”) promoting the hydraulic fracture well connection of well pairs as the new development that would commercialize trona solution mining. The application of hydraulic fracturing for trona solution mining was found to be unreliable. Fracture communication attempts failed in some cases and in other cases gained communication but not in the desired manner. The fracture communication project was abandoned in the early 1990's. Such hydraulic fracturing of trona has been proposed, claimed or discussed in numerous patents such as U.S. Pat. No. 2,847,202 (Pullen); U.S. Pat. No. 2,919,909 (Rule), among others.
At the present time, trona from the Green River basin is mainly produced by conventional mining methods described above, however, as part of a underground tailings disposal projects, operators have flooded old workings, dissolving the pillars and recovering the dissolved sodium value. This process was pioneered by FMC and described by their staff in the April 1999 E&MJ article “FMC's New Soda Ash Technology Is A Success”. The tailings disposal slurry deposits the solids in the old underground workings and the clarified solution dissolves some of the trona remaining in the mine pillars. The solution is then pumped to surface and evaporated, steam stripped to convert some of the bicarbonate to carbonate in solution, the remaining bicarbonate ions are converted to carbonate by a lime reaction, a solid phase soda ash decahydrate is recovered and the depleted solution is returned to the underground tailings disposal system. The soda ash decahydrate becomes an alternative feed stock to the existing monohydrate process plant providing the commercial soda ash products. This process is described in U.S. Pat. No. 5,283,054 issued in February 1994 to Copenhafer, Smith and Niedringhans. Individually, the process steps of this patent (evaporation to concentrate the solutions, steam stripping to convert some of the bicarbonate to carbonate, lime process (hydroxide process) conversion of the remaining bicarbonate ions to carbonate and the decahydrate process to recover soda ash values) are well-known processes.
Directional drilling from the ground surface has been used to connect dual wells for solution mining bedded evaporite deposits and the production of sodium bicarbonate, potash and salt. Directional drilling is used for solution mining bedded nahcolite deposits (naturally occurring sodium bicarbonate) and is described in Day, R. 1994 “White River Nahcolite Solution Mine,” Society for Mining, Metallurgy, and Exploration Meeting and Exhibit, February 1994, Albuquerque, N. Mex. Development of nahcolite solution mining cavities by using directionally drilled horizontal holes and vertical drill wells is described in U.S. Pat. No. 4,815,790, issued in 1989 to E. C. Rosar and R. Day, entitled Nahcolite Solution Mining Process and in United States Statutory Invention Registration No. H614, entitled “Method to Connect Drill Holes Utilizing Signaling Devices” to Robert Norman. The use of directional drilling for trona solution mining is described in U.S. Patent Application Publication No. US 2003/0029617 entitled “Application, Method and System For Single Well Solution Mining” by N. Brown and K. Nesselrode.
However, most trona deposits occur in narrow layered beds where the resource is disposed in beds that extend primarily horizontally with discrete boundaries of nonsoluble or nonevaporite rock disposed in between the beds. This bed configuration is costly to mine by conventional mining methods but this remains the dominant trona mining method. The practice of solution mining pillars does not materially improve the overall high soda ash production cost due to the high cost initial trona mining. There has been no known experimental or commercial activity to apply the directional drilling solution-mining methods known to the art.
Trona and nahcolite are the principle source minerals for the United States sodium bicarbonate industry. Sodium bicarbonate is produced by water dissolution and carbonation of either mechanically mined trona ore or the soda ash produced from that ore. Sodium bicarbonate is also produced by solution mining nahcolite, the naturally occurring form of sodium bicarbonate. Nahcolite solution mining utilizes directionally drilled boreholes and a hot aqueous solution comprised of dissolved soda ash, sodium bicarbonate and salt. In either case, the sodium bicarbonate is produced by cooling or a combination of cooling and evaporative crystallization.
Uranium mining is often by in situ leach methods (also known as solution mining). Uranium deposits, suitable for solution mining, generally occur in permeable sandstones, confined above and below by impermeable strata and below the water table. A wellfield design is typically a grid with alternating production and injection wells drilled vertically from surface. The well grid is typically drilled such that there is a borehole, drilled from the surface, in a center position. Fluid is injected into the permeable strata from this center borehole. Ranged around the center borehole are a number of boreholes, also drilled from the surface, from which fluid that has passed through the permeable strata is collected and recovered. This fluid contains dissolved uranium minerals that may then be recovered.
However, there remains a need in the art for improved methods of solution mining for evaporite minerals, in particular trona, and improved methods of solution mining non-evaporite soluble ores such as those containing uranium, and improved methods of producing coal, tar sands, heavy oil and oil shale. There remains a need in the art for solution mining methods that enhance resource recovery from evaporite mineral or ore beds, allow for recovery of a solvent that is rich in desired dissolved minerals and lean in undesired dissolved minerals leading to more cost effective winning of commercial products from the solvent, and reduced environmental impacts compared to conventional underground mining. There also remains a need in the art for underground configurations which allow for efficient storage and/or disposal of solids, gases and/or liquids; water wells; and containment systems/recovery systems for plumes of underground contaminants as well as for production of shale oil, heavy oils, tar sands and enhanced recovery from depleted conventional oil fields.